A variety of prior art technologies use adsorption processes for separating chemical compounds from gas flows, by passing such flows through a non-cohesive adsorbent material, typically activated carbons, macroporous resins, zeolites, silica gels, activated alumina or other types of molecular sieves.
These prior art technologies, resulting from the development of adsorption applications, may be divided into three base types, i.e. a first type based on discontinuous fixed beds, a second type based on continuously fluidized beds, a third type based on continuously rotating beds, as well as a number of hybrid technologies resulting from the integration of these basic technologies with others used upstream or downstream.
In view of providing solutions to the environmental problems caused by the increasing air pollution, environment protection regulations define increasingly restrictive contamination limits, wherefore the need arises for highly effective and reliable technologies.
Also, further directives, particularly concerning industrial processes, encourage research and use of technologies that involve a lower power consumption impact and a maximized recovery of reusable raw materials.
Referring to the above mentioned environment protection and economic issues, fluidized-bed and rotary-bed adsorption technologies may provide poor results in terms of effectiveness, efficiency and versatile operation, unless they are integrated with other technologies.
Particularly, a rotary bed, i.e. a compact apparatus that requires a smaller amount of adsorbent material due to intensive use thereof, is usually only effective as a first concentration stage for concentrating the volatile compounds contained at low concentration in gas flows, thereby generating a secondary gas flow, typically concentrated to 10-20 times, requiring a further separation for compound recovery, e.g. in a downstream fixed-bed adsorption system.
Patent application EP 1 683 563 A2 by the applicant hereof, discloses a continuous, rotary-bed gas purification system.
In this type of system, the adsorbent material is contained in a continuously rotating hollow disk-shaped rotor.
A plurality of gas flows simultaneously and pass through different sectors of such rotor.
One sector is passed through by the gas flow to be purified during adsorption, one sector is passed through by a first stabilized hot gas flow during desorption (for adsorbent material regeneration and recovery of desorbed compounds), one sector is passed through by a second stabilized desorption gas flow (for removal of the last compound traces), in series with the former, and one sector is passed through by a cooling gas flow.
The above process steps are cyclically repeated.
Typically, one operating cycle lasts from 0.25 h to 1 h.
Under the same treatment capacity conditions, the system of the above patent application requires a smaller amount of adsorbent material, due to its intensive use thereof, as well as lower investment costs, lower installation space requirements and a more regular consumption of utilities.
Particularly, the secondary gas flow is inert and contains the desorbed compounds at concentrations 100 to 200 higher than the primary incoming flow, wherefore such compounds may be conveniently recovered by simple condensation.
Other rotary-bed adsorption technologies are disclosed by many other patents.
Nonetheless, the mechanical complexity of rotary-bed systems is acceptable as long as the rotor has a small size, as related to the flow rates and the amount of compounds to be separated.
Fixed-bed adsorption technologies are still the most commonly used, in spite of their drawbacks in terms of investment and running costs, irregular consumption of utilities, operation safety problems, and dimensions, as related to the flow rates to be handled.
Here, continuous flow purification requires at least two fixed beds, which are alternately operated for adsorption or regeneration, each operating step lasting many hours (typically 4-12 h).
A number of patents disclose technological variants based on fixed-bed adsorption.
For instance, patent EP 1 492 610 B1 (U.S. Pat. No. 7,294,173 B2) by the applicant hereof, discloses a gas flow purification system which uses fixed beds with alternate adsorption and desorption steps. The desorption step is carried out by alternating bed desorption cycles, by circulation of hot inert gas at atmospheric pressure, with high-vacuum desorption cycles, for removing the last traces of polluting compounds.
The purpose of the above patent is to achieve superior purification effectiveness to meet the most restrictive requirements, as well as maximum operating safety.
Also, the need still exists for adsorption technologies which both ensure the high effectiveness of fixed-bed systems and have short cycles, for their operation to be as close as possible to continuous operation, using compact apparatuses, with regular consumption of utilities, and hence with all typical advantages of rotary systems.
The achievement of these purposes introduces a number of process and cost complications, which are particularly associated with the simultaneous distribution of multiple flows, having different and incompatible characteristics, to multiple adsorbent material containers, which complications become increasingly apparent as process flow rates increase.
Many patents provide alternative solutions to the problem of ensuring distribution of multiple flows to multiple adsorbent material containers, as used for the various steps of fixed-bed liquid or gas phase adsorption processes, which solutions rely on the use of rotary devices instead of complex valves and piping systems and their respective arrangement systems.
U.S. Pat. Nos. 1,387,857; 1,602,500; 2,312,941; 3,489,178; 4,469,494; 4,469,494; 4,968,334; 5,057,128; 5,112,367; 5,133,784; 5,248,325; 5,256,174; 5,632,804; 5,681,376; 5,779,771; 5,807,423; 5,814,131; 5,891,217; 6,063,161; 6,143,056; 6,253,778; 6,936,091 may be mentioned as an example.
The object of the present invention is to at least partially obviate the process and operational problems of the prior art, by providing an apparatus that is specially conceived to obviate the above construction and dimensional shortcomings, as well as a particularly effective, safe and reliable adsorption method, to be carried out using such apparatus.
In view of achieving such advantages to some extent, the method provides intensive use of adsorbent material with special characteristics, by dividing it into a larger number of smaller beds, and simultaneously performing adsorption and regeneration steps thereon in shorter operating cycles, using said apparatus and thus providing a semi-continuous process.